This invention relates to a protective coating composition. More particularly, it relates to a silicone resin coating composition which, when applied to a substrate, forms a protective, abrasion-resistant coating having improved crack resistance.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and/or ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions.
While these aforementioned coating formulations have been found acceptable, there still remains room for improvement. For example, the coating compositions of this invention provide coatings having improved resistance to moisture and humidity and ultraviolet light, in comparison with those coatings provided in accordance with U.S. Pat. No. 3,986,997. Moreover, it has been found herein that in direct contrast to the teachings of U.S. Pat. No. 3,986,997, the basic coating compositions of this invention having a pH in the range of from 7.1-7.8, do not immediately gel and provide excellent coatings having good properties on solid substrates. Furthermore, the compositions provided herein, while still possessing all of the advantages disclosed in copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978, possess improved resistance to deleterious cracking.
Protective coatings for metals, bright or dull, are also needed. For example, bright metallized plastics, wherein small amounts of metal are vacuum sputtered or vacuum metallized over the plastic, now popular with the auto industry because of their light weight, need protective layers to prevent scratching and marring of the brilliant surface. Metal wheel covers (hub caps) also require protective coatings for their preservation and lasting beauty.